Shaped bodies comprising textured superconducting material and a process for their production

ABSTRACT

The invention relates to a process for producing a shaped body, in which a mixture of oxidic starting powders or a superconducting material, which comprises at least 30% by volume of platelet-shaped primary particles and has such a composition that a high-temperature superconducting material is formed on later, suitable thermal treatment, is comminuted by milling, shearing and/or rolling in such a way that the comminuted powder has a powder particle size distribution having a d 90  of ≦20 μm, and in which the powders which have been comminuted in this way are isostatically compacted by the dry bag method.

The invention relates to a process for producing shaped bodiescomprising textured superconducting material and the further processingof these shaped bodies to form massive high-temperature superconductingcomponents such as power leads or high-temperature superconductingstrips and wires by the oxide-powder-in-tube (OPIT) method.

Owing to the still too low current-carrying capacity of manyhigh-temperature superconducting components, their use is limited.Further development of such components is necessary so that highersuperconduction currents can flow through these components. To producehigh-temperature superconductors having a high current-carryingcapacity, it is necessary to optimize the high-temperaturesuperconducting material in respect of purity, phase purity, phasecomposition, degree of crystallization and orientation.

The shaping of longitudinal bodies is customarily carried out using coldisostatic pressing (CIP) by the wet bag CIP method in which the pressingforces act on all sides of the shaped bodies, so that only weak ornonuniform alignment of the particles occurs.

In the wet bag method, a thin-walled, flexible rubber or plastic mold isfilled with powder and, after being closed, introduced into a pressurevessel. Compaction to form shaped bodies is achieved by means of aliquid pressure transfer medium at a predetermined pressure. Tightdimensional tolerances as are required, in particular, for introductionof the shaped bodies into silver sheathing tubes in the OPIT methodcannot be achieved in this way, so that further shaping, e.g. by turningthe shaped bodies on a lathe, is necessary. This additional stepincreases the complication of the process and increases costs and alsoentails the risk of increasing the residual carbon content in theproduct, particularly in the case of small particle sizes, as a resultof interaction with the CO₂ in the surrounding atmosphere duringmechanical forming. However, it is especially small particles sizeswhich are desirable for the OPIT method.

Since pressure is applied on all sides in the wet bag method, uniformpretexturing of the platelet-shaped particles cannot be achieved.Following shaping, attempts are made to compensate for the nonuniformpretexturing by means of the mechanical treatment steps, e.g. extrudingand/or rolling in the case of strip and wire production. This causesmany flaws, cracks and fractures in and between the inorganic powderparticles. These lead to comparatively low current densities or requirelong annealing times to heal these defects because unimpaired currentflow can occur only in the case of oriented, directly adjoiningdefect-free grains of the high-temperature superconducting phase.

In uniaxial pressing of longitudinal shaped bodies, e.g. in a hydraulicpress, the platelet-shaped powder particles are essentially alignedperpendicular to the pressing direction and thus perpendicular to thelongitudinal direction of the shaped body. The texture is rotatedprecisely by 90° from the optimum alignment and has a particularlyadverse effect on the current flow. When the texture is alignedoptimally, the crystallographic (a,b) plane which is oriented parallelto the plane of the platelets and to the plane of maximumcurrent-carrying capacity is in the direction of the current flow in thecomponent. During the thermal treatments following shaping, thepreferred orientation is essentially retained.

When metal tubes are filled manually with oxidic powders and whencompaction is achieved by ramming rather than mechanical pressing, notonly the increased effort but also the inhomogeneity of the presseddensity and the low density of the shaped body are particularlydisadvantageous. This leads to low superconducting contents in thecomponent and to particularly low current carrying capacities.

EP-A-0 611 737 reports a process for producing superconductors based on(Pb,Bi)—(Sr,Ca)—Cu—O, in which an oxidic melt is first produced andquenched and the solid obtained is particularly finely comminuted.EP-A-0 396 581 describes a process for producing wires or strips ofsuperconducting materials, in which process the compaction of the oxidicpowder is carried out by wet bag cold isostatic pressing. WO-94/00886likewise discloses a process in which isostatic compaction is achievedby the wet bag method.

WO-96/21951 teaches a process for producing an elongated superconductingbody either by extrudation or by two successive isostatic compactionsteps between which a heat treatment step is carried out or by acombination of first an extrusion procedure and two successive isostaticcompaction steps between which a heat treatment step is carried out.

Further significant disadvantages of the high-temperaturesuperconducting shaped bodies of the prior art are that the shapedbodies pressed by the wet bag method do not have a sufficiently accurateshape, that this method of shaping is particularly labor intensive andtime consuming and that the process cannot be automated.

Saleable shaped blanks for further processing to produce massivehigh-temperature superconducting components such as power leads or toproduce high-temperature superconducting strip or wire therefore have tohave properties which can be transferred to the product by the user bymeans of comparatively few and/or short energy-saving thermal and/ormechanical treatments. In addition, the objective of the optimization ofthese blanks has to be to make the entire production process up to thefinished products as simple and inexpensive as possible while alsoachieving improved product properties.

It is an object of the invention to provide a process for producing asuperconducting material, by means of which an improved texture can begenerated in a simple manner so that the heat-treated shaped bodieswhich can be produced by the process have fewer defects which impair theelectrical properties and also have homogeneous compaction, a highdensity and a higher current-carrying capacity. A further object is topropose a method which is economical to carry out and can be used for anindustrial process. In particular, it is an object to provide blanks inthe form of un-heat-treated or heat-treated shaped bodies such thattheir subsequent further processing to produce massive high-temperaturesuperconducting components such as power leads or to producehigh-temperature superconducting strips and wires can be carried outparticularly simply, quickly and with low energy consumption compared tothe prior art.

These objects are achieved by a process for producing a shaped body, inwhich a mixture of oxidic starting powders or a superconductingmaterial, which comprises at least 30% by volume of platelet-shapedprimary particles and has such a composition that a high-temperaturesuperconducting material is formed on later, suitable thermal treatment,is comminuted by milling, shearing and/or rolling in such a way that thecomminuted power has a powder particle size distribution having a d₉₀ of≦20 μm, and in which the powders which have been comminuted in this wayare isostatically compacted by the dry bag method.

For the purposes of the present invention, a shaped body is a body whichhas been shaped and can have been compacted according to the inventionand may have been treated by means of further process steps such as heattreatment. This shaped body can accordingly consist essentially of amixture of oxidic starting powders, of a superconducting material or ofa heat-treated high-temperature superconducting material. The termshaped body is therefore retained in various stages of the productionprocedure.

The starting material can be a mixture of oxidic starting powders or asuperconducting material. If organic salts such as a mixture of aplurality of oxalates or if a mixture of inorganic compounds such ascarbonates or/and sulfates are used as starting material, it isnecessary to calcine these first before use in the process of theinvention, since calcination liberates very large amounts of gas whichcould destroy a pressed body on heating. During calcination of suchstarting materials, usually at temperatures in the range from 150 to700° C., the nonoxidic compounds are converted into individual oxidesand/or multiple oxides. On the other hand, the starting material usedcan also be a material which is already superconducting and can havebeen produced by heat treatment, melting or/and comminution and oftendoes not yet have any, or not a high proportion of, high-temperaturesuperconducting phases.

Even during calcination, the first low or/and high-temperaturesuperconducting phases can sometimes be formed, e.g. the low temperaturesuperconducting single-layered phase Bi₂Sr₂Cu₁O_(x). In general, thehigh-temperature superconducting two-layered phase Bi₂Sr₂Ca₁Cu₂O_(x′) isformed from this single-layered phase only on heat treatment at highertemperatures or/and for longer periods, e.g. in the temperature rangefrom 700 to 830° C. Finally, significantly longer heat treatment formsthe high-temperature superconducting three-layered phaseBi₂Sr₂Ca₂Cu₃O_(x″) which can be converted into high-current-carrying,well crystallized, virtually phase-pure material only over long heattreatment times of usually more than 50 hours or even more than 100hours in the temperature range of usually from 820 to 850° C. Apart fromthese superconducting phases, a plurality of non-superconducting phasessuch as alkaline earth metal cuprates and copper oxide are usuallyproduced. However, the phase sequence indicated here is more complicatedand is very dependent on the composition and its changes during theprocess and also on the process parameters. As regards the heattreatment of the powders and shaped bodies and also the repeated heattreatment and the overall temperature curve and also the further processsteps such as comminution of the superconducting material and/or rollingof the multifilament, there are many possible variations. The objectiveof the process procedure has to be to achieve as high as possible aproportion of high-temperature superconducting phases, if possible aphase purity of ≧90% by volume, preferably ≧95% by volume orparticularly preferably almost 100% by volume, of high-temperaturesuperconducting phases. A further objective of the process has to be toproduce a texture, a grain structure and individual grains such that ashigh as possible a current-carrying capacity is achieved.

The starting powder mixture or the superconducting material which isused according to the invention has to have such a composition that ahigh-temperature superconducting material is formed on later suitablethermal treatment. The formation of this heat-treated high-temperaturesuperconducting material can be carried out by means of one or more heattreatment steps, preferably at the end of the manufacture of massivecomponents such as power leads, or by means of one or more heattreatment steps in combination with, for example, the rolling of theshaped body introduced into one or more metal tubes, preferably at theend of wire manufacture by the OPIT method. The starting powder and thesuperconducting materials which can be used for the process of theinvention are known in principle. Preference is given to those which, ifdesired in a mixture, have a mean particle size in the range from 1 to20 μm, in particular from 1.5 to 15 μm, particularly preferably from 2to 10 μm, measured using a laser granulometer Master Sizer from MalvernInstruments after ultrasound dispersion. The starting powders and thesuperconducting materials are advantageously of high purity, with thesum of the metallic impurities being ≦1000 ppm, particularly preferably≦500 ppm. Their carbon content is preferably ≦500 ppm, in particular≦300 ppm, since this avoids bubble formation, for example in a wire.Bubbles have a strong adverse effect on the current-carrying capacityand the mechanical properties of the wire.

The starting powder mixture or the superconducting material which isused at the beginning of the process of the invention should comprise atleast 30% by volume, in particular at least 50% by volume, particularlypreferably at least 70% by volume, of platelet-shaped primary particles.The term primary particles is used to describe the smallest individualparticles, e.g. platelet-shaped particles, which can be achieved onappropriate comminution. It has been observed in initial experimentsthat the proportion of the high-temperature superconducting phase(s) isgenerally higher, the higher the content of platelet-shaped primaryparticles. The starting powder mixtures and the superconductingmaterials which may have been specifically comminuted usually contain ahigh proportion of particles consisting of partially sintered, hardaggregates as well as relatively easily separable agglomerates. Theaggregates and agglomerates often consist essentially of platelet-shapedprimary particles which are joined together strongly in the case of theaggregates and comparatively weakly in the case of the agglomerates(FIG. 1). The proportion of particles in the form of separateplatelet-shaped primary particles is generally low.

On comminution of the particles to a powder particle size distributionhaving a d₉₀ of ≦20 μm, the aggregates are largely destroyed, theagglomerates are very largely divided and, in the case of exceptionallyharsh comminution, many platelet-shaped primary particles are alsobroken up. The starting powder mixture or/and the superconductingmaterial is advantageously comminuted in such a way that at most 90% byvolume, in particular at most 70% by volume, more preferably at most 50%by volume, but particularly preferably at least 10% by volume and evenmore preferably at least 25% by volume, of the powder particles arepresent in platelet form for shaping. On comminution, the powders arepreferably milled down to an agglomerate particle size in which no orvirtually no undestroyed, hard aggregates >15 μm and if possible also noor virtually no comparatively readily destroyable agglomerates >25 μmstill occur. The optimum comminution according to the inventioncomprises separation of the primary particles on division of theaggregates and agglomerates without the platelet-shaped primaryparticles being milled to a great extent. The proportions of theplatelet-shaped primary particles recognizable under a scanning electronmicroscope often decrease significantly on milling, shearing or/androlling. If, on the other hand, the starting powder mixture or thesuperconducting material is treated thermally before or/and aftercomminution, the proportion of plateletshaped primary particlesgenerally increases, e.g. to at least 40% by volume, in particular to atleast 60% by volume, especially to at least 75% by volume, on heattreatment at from about 600 to 800° C. for preferably from 2 to 10 hoursin an oxygen-containing atmosphere.

Particularly if the platelet-shaped primary particles have been brokenup too much during comminution, it is advisable to carry out this heattreatment in order to restore the platelet shape of the primaryparticles by crystal growth. However, should this heat treatment havetoo great an effect and lead to somewhat strongly sintered powders,light milling is necessary, e.g. in a vibration mill for about 15minutes. It is advisable to monitor this process by means of examinationunder a scanning electron microscope.

The comminution of the starting powder mixture or the superconductingmaterial by milling, shearing or/and rolling is preferably carried outusing a vibration mill, a jet mill or/and shearing rollers. Here, drymilling with an organic liquid content of up to 0.5% by weight ispreferred to wet milling in organic solvents. Preferably, only organicmilling auxiliaries are added to the starting powder mixture or thesuperconducting material. These auxiliaries can, on the one hand, beuseful as milling aids during comminution but, on the other hand, canalso serve to produce free-flowing granular materials or/and readilypressable shaped bodies. When polymeric liquids such as surfactants areused, preference is given to adding amounts of up to 0.5% by weight.

Vibration milling preferably takes from 20 to 80 minutes, in particularfrom 30 to 60 minutes, at an amplitude of preferably from 3 to 6 mm, inparticular from 4 to 5 mm, when using milling media made of, forexample, zirconium oxide ceramic and a degree of fill with milling mediaof from 40 to 80%, preferably from 55 to 65%, and an amount of materialbeing milled which preferably corresponds approximately to theinterstitial volume between the milling media.

Jet milling can be carried out in a spiral jet mill using a powder feedrate of, for example, 500 g/h in one pass through the mill. The use ofshear rollers is particularly well suited to delaminating fine particleswhich comprise platelet-shaped primary particles or can be readilycleaved.

Use of the preferred comminution equipment enables contamination of themilled product by abraded material from the milling media to be largelyavoided when using plastic-lined milling vessels and as a result of theformation of a layer of material being milled on the surface of themilling media. Milling in an inert atmosphere such as technical-gradenitrogen makes it possible to limit the increase in the carbon contentof the material being milled to contributions of about 50 ppm in thecase of vibration milling and to contributions of from 50 to 100 ppm inthe case of a jet mill. Surfactants as milling aids can surprisinglycontribute the formation of a free-flowing granular material withoutspray drying or another granulation technique having to be employed.Free-flowing granular materials are preferable for automatic filling,but contents of polymeric liquids or/and other organic auxiliariesshould preferably be removed immediately after pressing or beforeintroduction of the pressed shaped body into a tube. At the latestbefore introduction of a shaped body into a metal tube, preferablyimmediately after pressing, the organic auxiliaries should be removed bybaking out, burning out or chemical desorption.

The shaped bodies preferably have an essentially elongated, cylindrical,prismatic or tube-shaped geometry, but can also have an essentiallyisometric, ring-shaped, disk-shaped or other geometry. The pressedshaped bodies can be solid bodies or hollow bodies. It is preferable forthe ratio of length to wall thickness of the shaped body at the time ofshaping to be at least 2:1, preferably at least 4:1, particularlypreferably at least 6:1, very particularly preferably at least 8:1. Thelength of the shaped bodies which are made from the pressed shapedbodies and are processed further according to the invention and the endproducts produced therefrom can be reduced in size, for example bydividing for a plurality of individual components or by machining off ofends, so that the ratio of the length to wall thickness which ispreferably adhered to during shaping does not have to be maintained inrespect of the component geometry. The shaped bodies are, in particular,blanks for the manufacture of wires, strips, rods, tubes or hollow orsolid bodies which can be used, in particular, for the production of anduse as high current cables, transformers, windings, magnets, magneticbearings or power leads. It is advisable to optimize the shaped bodiesso that they can be processed in a comparatively simple subsequentproduction process to give the desired high-temperature superconductingcomponents. These subsequent process steps include, in particular, oneor more firings in which the previously textured powder particles keep astrong texture during sintering.

In isostatic pressing by the dry bag method, preference is given tousing a pressing pressure of from 400 to 5000 bar, in particular from1500 to 3000 bar, particularly preferably from 1800 to 2500 bar, inorder to effect the compaction and texturing of the powder. If thecomminuted powder, in particular a granular material, is sufficientlyfree-flowing, it is possible to employ automatic powder metering, anautomatic pressing cycle and automatic removal of the compact so that ashaped body or, in the case of a multiple tool, also a correspondinglyhigher number of shaped bodies is obtained in a very short time (about 2minutes cycle time), which corresponds to about one tenth of the timefor the wet bag method.

FIG. 2 shows, by way of example, a schematic diagram of a tool block (1)of an isostatic press using the dry bag method with bag (2) and thecharge of material to be pressed (3). The pressing mold (4) in the drybag method generally has a significantly thicker wall than in the wetbag method and has a higher hardness, e.g. Shore A in the range from 80to 90. For example, it is possible to use a polyurethane bag which has athickness of about 30 mm and has high shape stability in thepressureless state, good elasticity when pressure is applied, goodrecovery properties on release of the pressure and good shape stabilityin long-term operation. In this way, improved dimensional accuracy andtrueness of shape are achieved. The bag (2) is installed in a tool block(1) and is subjected only radially to hydrostatic pressure via anelastic separating membrane (4) by means of a pressure transfer medium(5), e.g. one based on water. The construction of the press almostcompletely prevents non-radial pressing force components, since theclosing punches (6, 6 a) themselves transmit virtually no force but onlyact counter to the force component acting from the inside outward. Theeffect of this on the pressed shaped bodies is to introduce the desiredtexture. Owing to the construction of the press and the properties ofthe bag, tight shape and positional tolerances can be maintained. Theentire cycle of filling, closing, pressurization and depressurization,opening and removal of the shaped body can be carried out fullyautomatically.

When pressing hollow bodies, it is possible to arrange, for example, ametal core preferably centrically in the hollow space of the bag. Theshaped body pressed onto the core can easily be removed from the bagowing to the elastic recovery of the oxidic material if the nature ofthe composition is suitable or/and if conical cores are employed.

The desired texture is obtained if no or virtually no intact hardaggregates >15 μm are present in the material being pressed. Theagglomerates have, as a rule, been broken down during pressing but maybe further divided in a subsequent mechanical processing step. Thetexture is, for example in the case of suitable powders based on(Bi,Pb)—(Sr,Ca)—Cu—O, such that the texture does not changesignificantly from the edge to the middle of, for example, 10 mm thickshaped bodies under a scanning electron microscope and is comparativelystrongly aligned in the axial direction of the shaped body. Heattreatment does not have an adverse effect on the texture of the shapedbodies.

It has surprisingly been found that the combination of a comminution inwhich the hard aggregates are largely destroyed and the platelet-shapedprimary particles are not broken up very much with isostatic shaping bythe dry bag method using virtually completely radial force transmissionmakes it possible to produce well pretextured shaped bodies whichsignificantly reduce the expense of further processing, in particularfurther mechanical shaping or/and long heat treatment.

Particularly in the production of superconducting strips and wires, atleast one elongated metal body is preferably embedded in the shaped bodyduring shaping. The or every elongated metal body can be embedded as acore in the longitudinal direction in the region of the longitudinalaxis or around the longitudinal axis of the shaped body. The elongatedmetal body can be, inter alia, a rod, wire or filament and cancontribute to a higher current density in the product as a result ofboundary layer effects between superconducting ceramic and metal.Particularly in the production of superconducting strips and wires, theor every shaped body is preferably inserted into a tube having anessentially cylindrical or prismatic shape, in particular a preferablymetallic tube closed at one end. The or every elongated body or/and thetube consists essentially of at least one metal or at least one alloyand preferably comprises silver.

The shaped body has good ductility in the production of monofilament andmultifilament strips or wires by the powder-in-tube method for producinghigh-current conductors for electrical engineering. The blank can befinished after pressing the shaped body or after heat treatment of thepressed shaped body. The blank can be further processed by single orrepeated extrusion or/and rolling to produce a monofilament and, ifdesired, by bundling together a plurality of monofilaments and possiblyadditionally by single or repeated extrusion or/and rolling to produce amultifilament. Particularly in the production of superconducting stripsand wires, the shaped body, the tube filled with the shaped body/bodiesor the further processed shaped body is alternately heat-treated androlled a number of times. These process steps are known in principle.During these steps, an even better texture of the blank, for examplelocated in a silver sheath, should be achieved. Advantageously, heattreatments are carried out between the last rolling steps, which heattreatments are, for example in the case of materials based on(Bi,Pb)—(Sr,Ca)—Cu—O with or without a lead content, to effect thevirtually complete formation of the superconducting phase BSCCO 2223 andthe healing of the structural defects in the platelet-shaped primaryparticles introduced during extrusion/rolling. The compaction, thedegree of texturing, the frequency of defects and the phase compositionand distribution of the superconducting phase(s) are decisiveinfluencing factors on the current-carrying capacity of thesuperconducting materials and on the currents to be carried by thecomponents. The strips and wires produced in this way have aconsiderably higher proportion of high-temperature superconductingmaterial in relation to the proportion of metal(s). The sheathing factoras ratio of the proportion of ceramic to the total cross section of thestrip or wire is thus considerably increased, like the current-carryingcapacity.

It is necessary for the shaped body, the tube filled with the shapedbody/bodies or the further processed shaped body to be heat-treated attemperatures of from 400 to 900° C., in particular from 500 to 850° C.,and in the case of materials based on (Y,RE)—Ae—Cu—O even in the rangefrom 400 to 1100° C., preferably from 600 to 1050° C. Here, Aedesignates alkaline earth elements. In the case of indications of thesubstitution of elements, all types of elements shown in brackets do notalways have to be present. To produce massive components based on(Bi,Pb)—Ae—Cu—O, heat treatment at, for example, from 800 to 850° C. forfrom 10 to 150 hours, preferably for from 20 to 100 hours, in particularfor from 30 to 60 hours, in an oxygen-containing atmosphere isnecessary. It is advisable for the atmosphere to consist essentially ofa mixture of nitrogen and oxygen containing from 0.1 to 50% by volume,in particular from 1 to 30% by volume, particularly preferably from 10to 25% by volume, of oxygen. Preference is given to working in a streamof gas. The heat-treated shaped body can be subjected to at least onefurther heat treatment.

The superconducting material or the heat-treated high-temperaturesuperconducting material comprises at least one of the superconductingphases having a composition based essentially on (Bi,Pb)—Ae—Cu—O,(Y,RE)—Ae—Cu—O or (Tl,Pb)—(Ae,Y)—Cu—O, where Ae designates alkalineearth elements and in particular Ba, Ca or/and Sr. Here, the phaseswhich occur have, in particular, a composition of approximately(Bi,Pb)₂(Sr,Ca)₂Cu₁O_(x′), (Bi,Pb)₂(Sr,Ca)₃Cu₂O_(x″),(Bi,Pb)₂(Sr,Ca)₄Cu₃O_(x′″), (Y,SE)₁Ba₂Cu₃O_(Y′), (Y,SE)₂Ba₁Cu₁O_(y″),(Tl,Pb)₂(Ba,Ca)₂Cu₁O_(z′),(Tl,Pb)₂(Ca,Ba)₃Cu₂O_(z″),(Tl,Pb)₂(Ca,Ba)₄Cu₃O_(z′″), (Tl,Pb)₁(Ca,Ba)₃Cu₂O_(z″″),(Tl,Pb)₁(Ca,Ba)₄Cu₃O_(z′″″). In some cases it is advisable to mixsulfate(s) into the starting powder mixture, the superconductingmaterial or the heat treated high-temperature superconducting materialso that alkaline earth metal sulfates, in particular BaSO₄, SrSO₄ or/and(Ba,Sr)SO₄, are present in addition to the superconducting phase(s).

If the superconducting shaped body of the invention has been subjectedto only moderate heat treatment, its predominantly platelet-shapedprimary particles have grown compared to those of the pressed shapedbody and are partly sintered together and, at least in part of the outerwall, are aligned essentially perpendicular to the pressing direction ofthe isostatic compaction step.

If the superconducting shaped body of the invention has been stronglyheat-treated, its microstructure is strongly sintered together and, atleast in part of the outer wall, is aligned essentially perpendicular tothe pressing direction of the isostatic compaction step.

The shaped body of the invention can be used as a blank, preferably forthe production of high-temperature superconducting wires, strips, rods,tubes, hollow or solid bodies, in particular for producing high currentcables, transformers, windings, magnets, magnetic bearings or powerleads.

FIGURES

FIG. 1 shows a scanning electron micrograph of an oxidic starting powderbased on (Bi,Pb)—Ae—Cu—O before milling as described in claim 1.

FIG. 2 shows a schematic diagram of a tool block of an isostatic pressusing the dry bag method with bag and charge of material to be pressed.

EXAMPLES

The invention is illustrated by means of the following comparativeexample and the examples according to the invention:

Comparative Example 1:

A powder having the compositionBi_(1.7)Pb_(0.33)Sr_(1.88)Ca_(1.96)Cu₃O_(x) was synthesized withoutcomminution in a plurality of heat treatment steps over a total of 20hours at 790° C. until it had been converted into the two-layermaterial, with the high-temperature superconducting phase BSCCO 2212accompanied by certain amounts of copper oxide and alkaline earth metalcuprates being present. The material obtained in this way had a carboncontent of 200 ppm and a nitrogen content of 100 ppm. Theplatelet-shaped primary particles mostly had a thickness of from 0.3 to0.5 μm and a length of from 1 to 8 μm, as determined on fracturesurfaces under a scanning electron microscope. The mean size of theparticles, which were mostly present in the form of agglomerates andaggregates, was about 20 μm; it was reduced to 13 μm by treatment withultrasound from an ultrasonic probe for fifteen minutes in thepreparation of the suspension for determination of the particle sizedistribution by means of a laser granulometer Master Sizer XSB.OD fromMalvern Instruments. The powder was, without comminution or granulationbeing carried out after heat treatment, isostatically pressed at 2500bar by the wet bag method to produce cylinders having a diameter of 12±2mm and a length of 110 mm. The shape deviation of the compacts forfurther processing was so great that the shaped body could be introducedinto a silver sheathing tube and had the required accuracy of fit of atmost ±0.2 mm diameter fluctuation only after mechanical shaping byturning. The density of the compacts was 4.3 g/cm³, which corresponds toa relative density of about 68%. The intact, hard aggregates can berecognized by means of a scanning electron microscope even afterpressing. No noticeable alignment of the particles was visible under thescanning electron microscope. The particles were therefore not orientedessentially in the direction of the future current flow.

Example 1:

The starting powder of Comparative Example 1 was used and dry milled for60 minutes to a mean particle size of 4 μm in a vibration mill model M18from SWECO Europe. In all the examples according to the invention, theparticle size distribution was determined using the laser granulometerwithout ultrasound treatment. The milled material was isostaticallypressed at 2000 bar by the dry bag method in a tool corresponding toFIG. 1 under conditions otherwise identical to those in the comparativeexample to produce shaped bodies having a diameter of 12±0.2 mm and alength of 110 mm. The diameter tolerance of +0.2 mm could be easilyachieved in a large number of specimens. The green density was 4.5g/cm³, which corresponds to a relative density of about 71%. Aggregatescould no longer be recognized on fracture surfaces of the pressed shapedbodies (=blanks) using a scanning electron microscope. The platelettexture in the axial direction was very pronounced.

Example 2:

The starting powder of Comparative Example 1 was used and dry milled toa mean particle size of 2.5 μm in a laboratory jet mill model MC100KXBDfrom Crispo AG at a throughput of 500 g/h and one pass through the mill.The milled material was isostatically pressed by the dry bag methodunder the same conditions as in Example 1 to produce shaped bodieshaving the same dimensions as in Example 1. Their green density was 4.45g/cm³, which corresponds to a relative density of about 70%. Aggregatescould no longer be recognized on fracture surfaces of pressed shapedbodies under a scanning electron microscope. The platelet texture in theaxial direction was clearly pronounced.

Example 3:

A powder mixture having the compositionBi_(1.9)Sr_(2.11)Ca_(0.9)Cu₂O_(Y) was homogeneously melted at atemperature of about 1000° C. The melt was poured into a mold. Aftercooling to room temperature, the solid was comminuted roughly. Thispowder was dry milled to a mean particle size of 3 μm in a milling andscreening unit model 100AFG from Hosokawa-Alpine AG, subjected tointermediate heat treatment to further reduce the carbon content whichhad been increased during milling and isostatically pressed at 2000 barusing the dry bag method to produce prismatic shaped bodies havingdimensions of 9^(−0.2) mm×9^(−0.2) mm×100 mm. The density was 4.7 g/cm³,which corresponds to a relative density of about 71%. Examination undera scanning electron microscope indicated excellent texture in the axialdirection.

Example 4:

The procedure of Example 1 was repeated under the same conditions,except that 0.01% of an organic liquid in the form of a solvent-freeacid was added to the starting powder. The same results as in Example 1were obtained, although the milled powder was granular and free-flowingso that it was well suited to pressing using automatic charging,metering and pressing.

What is claimed is:
 1. A process for producing a shaped body, in which a mixture of oxidic starting powders or a superconducting material, which comprises at least 30% by volume of platelet-shaped primary particles and has such a composition that a high-temperature superconducting material is formed on later, suitable thermal treatment, is comminuted by milling, shearing or rolling in such a way that the comminuted power has a powder particle size distribution having a d₉₀ of ≦20 μm, and in which the powders which have been comminuted in this way are isostatically compacted only by radial pressure by the dry bag method.
 2. The process of as claimed in claim 1, wherein the starting powder mixture or the superconducting material is comminuted in such a way that at most 90% by volume of the powder particles are present in platelet form for isotatic pressing.
 3. The process of as claimed in claim 1, wherein the starting powder mixture or the superconducting material is comminuted using a vibration mill, a jet mill or shear rollers.
 4. The process as claimed in claim 1, wherein the starting powder mixture or the superconducting material is thermally treated before or after comminution so that the proportion of platelet-shaped primary particles is increased in the range to at least 40% by volume.
 5. The process as claimed in claim 4, wherein the proportion is increased to at least 60% by volume.
 6. The process as claimed in claim 5, wherein the proportion is increased to at least 75% by volume.
 7. The process as claimed in claim 1, wherein organic auxiliaries are added to the starting powder mixture or the superconducting material and comminution in the dry or virtually dry state produces a free-flowing powder.
 8. The process as claimed in claim 1, wherein the comminuted powder is shaped to produce essentially cylindrical, prismatic or tubular shaped bodies.
 9. The process as claim in claim 8, wherein at least one elongated metal body is embedded in the shaped body during shaping.
 10. The process as claimed in claim 9, wherein the elongated metal body is embedded as a core in the longitudinal direction in the region of or around the longitudinal axis of the shaped body.
 11. The process as claimed in claim 9, wherein the longitudinal body or the tubular shaped body consists essentially of at least one metal or at least one alloy and comprises silver.
 12. The process as claimed in claim 8, wherein the shaped body is introduced into a tube having an essentially cylindrical or prismatic shape, including a metallic tube closed at one end.
 13. The process as claimed in claim 1, wherein the comminuted powders are shaped to produce longitudinal shaped bodies having a ratio of length to external diameter or of length to greatest thickness of at least 2:1, in particular at least 4:1.
 14. The process as claimed in claim 13, wherein the shaped body is further processed by single or repeated extrusion or rolling to produce a monofilament and, if desired, by bundling together a plurality of monofilaments and possibly additionally by single or repeated extrusion or rolling to produce a multifilament.
 15. The process as claimed in claim 14, wherein the shaped body, the tube filled with the shaped body/bodies or the further processed shaped body is heat-treated at temperatures of from 400 to 1100° C.
 16. The process as claimed in claim 15, wherein the shaped body, the tube filled with the shaped body/bodies or the further processed shaped body is heat-treated by means of at least one intermediate or subsequent thermal treatment.
 17. The process as claimed in claim 16, wherein the shaped body, the tube filled with the shaped body/bodies or the further processed shaped body is alternately heat-treated and rolled a plurality of times.
 18. The process as claimed in claim 17, wherein the superconducting material or the heat-treated high-temperature superconducting material comprises at least one of the superconducting phases having a composition based on (Bi,Pb)—Ae—Cu—O, (Y,RE)—Ae—CU—O or (TI,Pb)—(Ae,Y)—Cu—O, where Ae designates alkaline earth elements, Ba, Ca or Sr.
 19. The process as claimed in claim 18, wherein sulfate(s) is/are mixed into the starting powder mixture, the superconducting material or the heat-treated high-temperature superconducting material so that not only the superconducting phase(s) but also alkaline earth metal sulfates, of BaSO₄, SrSO₄, or (Ba,Sr)SO₄, are obtained.
 20. The process as claimed in claim 1, wherein a multiple tool is used for isostatic compaction.
 21. The process as claimed in claim 1, wherein the thermal treatment includes baking out, burning out or chemical desorption to remove organic auxiliaries. 